Our work on muscarinic acetylcholine receptors focuses on two fundamental questions: (1) How is binding of acetylcholine to receptors converted into an intracellular response? (2) What factors regulate receptor concentrations in the membrane? A binding assay employing tritiated quinuclidinyl benzilate permits us to investigate properties of receptor binding sites and to measure receptor levels in cultured neuroblastoma and hybrid cells under systematically varied conditions.